Figure 4.

SWI consists of the processing of complex gradient echo MRI data, i.e. its phase (a) and magnitude components (b). Here, we used the fourth echo from a multi echo gradient echo acquisition (TE = 26.7 ms, TR = 32.0 ms, voxel size = 0.65×0.65×1.4 mm³). The phase (a) is high-pass filtered in order to remove the background phase (c). Contrast is generated by variations in the magnetic field generated by local susceptibility sources only. We can note that paramagnetic materials (haemorrhage, venous blood) appear as negative phase shifts, while diamagnetic materials (calcification) appear as positive phase shifts. The filtered phase data is further processed to generate a phase mask (d) that only darkens areas where the phase shift is negative. The magnitude image is subsequently multiplied n times (here n = 4) in order to generate the SWI (e). The phase mask was chosen to suppress signals from negative phase shifts only, so that the visibility of both haemorrhages and the venous vasculature was enhanced. A mIP may be employed to further emphasize paramagnetic venous vasculature across the stack of slices (f). mIP,minimum intensity projection; SWI, Susceptibility Weighted Imaging; TE, echo time; TR, repetition time.